This invention is directed to improving the adhesion of nylon coatings on substrates of nylon and polyester. More specifically this invention is directed to the use of select formaldehyde resins with high imino content or partially alkylated or non-alkylated derivatives thereof to improve the adhesion of mixed nylon polymers to filaments, films, parts and the like made from nylons and polyesters.
Nylon xe2x80x9cmulti-polymersxe2x80x9d are nylons made from a mixture of nylon forming monomers such that the nylon polymer contains a mixture of at least two types of nylon structural units. These types of nylons are sold commercially for a variety of coatings and adhesive applications. Generally these nylons are readily soluble in organic solvents and are generally applied as solutions. See for example brochures entitled xe2x80x9cElvamide(copyright) Nylon Multipolymer Resins, Properties and Usesxe2x80x9d (September 1977) and xe2x80x9cElvamide(copyright) Nylon Multipolymer Resins for Thread Bondingxe2x80x9d (October 1977), both published by E.I. DuPont de Nemours and Company, Inc.
Typically, sewing threads are coated with polymeric materials (and with lubricants added in most cases) to protect them from abrasion during the sewing operation. Furthermore, with twisted multi-filament sewing threads the polymeric coatings (also referred to as thread bonding) also prevent fraying and unraveling (untwisting) of the individual filaments. See generally, the December 1990 DuPont brochure and Kohan, M. I., xe2x80x9cNylon Plastics Handbookxe2x80x9d Hansen/Gardner Publications, Inc. (1995) pages 283-290.
Nylon multi-polymers have been used for thread bonding applications for several decades. However, there is increasing demand for improved adhesion of the coating to the thread, as for example in highly demanding modern applications. This is also of paramount importance for applications relying upon tightly woven fabric, for example in luggage and automotive air bags, leather, and the like. In such applications abrasion of the thread is high and the operating needle temperature is much higher compared to more loosely woven fabric applications such as those used in apparel. The poor adhesion of coatings results in a xe2x80x98snake skinxe2x80x9d effect where the coating comes off the surface of the thread. This results in loose coating material that jams the needle requiring stoppage of the operation. Further, poor aesthetics are associated with loose coating material as can be seen on inspection of the surface of the thread.
In the case of sewing threads and fabrics, nylon copolymers, terpolymers, and higher multi-polymers are used for coatings applications. These polymers are usually soluble in organic solvents, especially alcohols. The nylon coating is typically applied by dipping the thread in a solution of the nylon multi-polymer and then subsequently passing the thread through a drying chamber and then to a fusing chamber generally at a temperature above the melting point of the nylon mixed polymer. Melting of the nylon multi-polymer coating on the thread promotes adhesion. Nylon mixed polymers are generally favored for this use because of their toughness, good abrasion resistance, and ready solubility in solvents. For more information on these procedures and the benefits associated with nylon mixed polymers, see the Elvamide(copyright) (October 1977) brochure and the xe2x80x9cNylon Plastics Handbookxe2x80x9d mentioned above.
The brochures mentioned above describe the ability of thermosetting resins such as epoxy, phenol-formaldehyde, and melamine-formaldehyde to cross-link nylon multi-polymers and improve the adhesion of the coating. The nylon multi-polymer reacts with these thermosetting resins to form thermoset-thermoplastic compositions.
U.S. Pat. No. 4,992,515 describes the use of Cymel(copyright) 1135 available from Cytek Industries, Inc., a fully alkylated melamine-formaldehyde resin, and strong acid catalyst to cross-link nylon 6/66/69, nylon 6/66/610, and nylon 6/66/612 terpolymers. The extent of cross-linking achieved was measured by the insolubility in the original solvent of the coating material after cross-linking. The cross-linked nylon coating becomes insoluble in the solvent. However, no data was provided as to how much improvement in adhesion and resistance to unraveling were achieved.
Various types and re-activities of formaldehyde derived cross-linking agents are disclosed in a brochure entitled xe2x80x9cHigh Solids Amino Crosslinking Agentsxe2x80x9d (September 1994) available from Cytec. For example, Cymel(copyright) 325 used in several examples described below has free formaldehyde of 1.0 weight percent. Other grades of Cymel(copyright) can contain up to 3.5 weight percent free formaldehyde and are useful in the practice of the instant invention. These and other cross-linking agents are prepared by the reaction of amine functionalities with formaldehyde resulting in the replacement of the hydrogen on the amine functionality by a hydroxymethyl group. The hydroxymethy function is reacted with an alcohol to convert the hydroxy function to an alkoxy. Many classes of these crosslinking agents are possible depending on the extent of reaction. For example, there are commercially available types in melamine-formaldehyde cross-linking resins. Partial reaction of the amino functionalities in melamine (Structure 1 below) 
results in Structure 2
where only some of the hydrogens have been replaced by the hydroxymethyl groups. Alkylation reaction of Structure 2 with an alcohol results in the conversion of the hydroxy group to alkoxy group as shown in Structure 3. 
Melamine-formaldehyde resins containing the type of functionality as in Structure 3 are classified as high imino-type resins. Complete replacement reaction of melamine with formaldehyde and subsequent partial alkylation results in Structure 4. 
Again resins containing this type of functionality are classified as partially alkylated. If the reaction with alcohol is allowed to reach completion the fully alkylated derivative (Structure 5) 
is obtained. Those having skill in the art will readily appreciate that different classes of functionalities (eg, amino or alkoxy groups) may be designed into the molecule. Each class is chemically distinct and has different characteristics and re-activities. The fully alkylated resins such as Cymel(copyright) 1135 require catalysis by strong acids to initiate their reaction.
There is a longstanding need for a technique to improve the overall adhesion of nylon coatings to substrates of nylon and polyester. Improvements in such adhesion will promote better aesthetic qualities to the article formed, and also provide an economic benefit in that less material is rejected as nonconforming for the intended final product.
An object of the instant invention is to develop a processing technique and coating solutions to improve the adhesion of nylon coatings to nylons, polyesters, and mixtures thereof. This development applies not only to threads but in general to any substrates where such adhesion is desirable. A further object of the instant invention is to provide such techniques and solutions that are readily adaptable and useful for a variety of applications including monofilaments, multifilaments, films, tubings, shaped parts and the like. A feature of the present invention is the durability of the adhesive bond itself making it suitable for rigorous applications in which the material is extensively handled and manipulated. An advantage of the present invention is that the procedure may utilize any of a variety of solvents. These and other objects, features and advantages of the invention will become better understood upon having reference to the following descriptions of the invention.
Coating solutions to promote the adhesion of polyamides to substrates of polyamides, polyesters or mixtures thereof are disclosed herein. These solutions comprise:
(a) polyamide having a solubility of at least 0.5 weight percent in select organic solvents, and
(b) 1 to 100 weight percent based on the weight of the polyamide of high imino, partially alkylated or non-alkylated formaldehyde resins selected from the group consisting of melamine-formaldehyde, glycoluril-formaldehyde, benzoguanamine-formaldehyde, and mixtures thereof.
Optionally, 0-20 weight percent based on the weight of the formaldehyde resin of a catalyst may be added. Additionally, fully alkylated melamine-formaldehyde, glycoluril-formaldehyde, or benzoguanamine-formaldehyde resins may be added. The resins (b) function as adhesion promoters. The resins (b) are preferably incorporated in the range of 1-40 weight percent (most preferably 1-20 weight percent) based on the weight of the polyamide.
There are also disclosed herein processes for the coating of these substrates with the coating solutions of the invention. Such processes are readily appreciated by those having skill in the art. See for example the thread coating procedures described herein.